Novel electro deposited gold; electrolyte solution to obtain the same; method for producing said gold; and products

ABSTRACT

An aqueous electrolyte solution having a pH from 5.5 to 8 for depositing hard ductile and bright arsenic containing gold has been provided. Besides the buffers and the alkali gold cyanide complex, a thio sulfate or an organic dithio compound is added to facilitate the proper inclusion of arsenic in the gold deposit. Salt compositions suitable for obtaining aqueous electrolytes; a method for depositing the arsenic gold; the gold alloy; and electrical devices having the arsenic gold deposit on a surface of these devices have also been disclosed.

United States Patent [191 Zimmerman et al.

[451 July 29, 1975 NOVEL ELECTRO DEPOSITED GOLD;

ELECTROLYTE SOLUTION TO OBTAIN THE SAME; METHOD FOR PRODUCING SAID GOLD; AND PRODUCTS [75] Inventors: Richard Henry Zimmerman,

Hershey; Richard Lee Brenneman, Harrisburg, both of Pa.

[73] Assignee: AMP Incorporated, Harrisburg, Pa.

[22] Filed: Aug. 13, 1973 [21] Appl. No.: 387,870

Related US. Application Data [60] Continuation of Ser. No. 208,394, Dec. 15, 1971, abandoned, which is a division of Ser. No. 807,105, March 13, 1969, abandoned.

[52] U.S. Cl 75/165; 29/199 [51] Int. Cl. C22c 5/00 [58] Field of Search 75/165, 134 P; 29/199;

[56] References Cited UNITED STATES PATENTS 3,423,295 l/l969 Greenspan 204/43 G 7/l970 Duva 204/43 G OTHER PUBLICATIONS M. Hansen, Constitution of Binary Alloys N.Y., McGraw-Hill, 2nd ed., 1958, pp. l54-l55.

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney, Agent, or FirmGerald K. Kita [57] ABSTRACT 4 Claims, No Drawings NOVEL ELECTRO DEPOSITED GOLD; ELECTROLYTE SOLUTION TO OBTAIN THE SAME; METHOD FOR PRODUCING SAID GOLD; AND PRODUCTS This is a streamlined continuation of application Ser. No. 208,394, filed Dec. 15, I971, abandoned, which is in turn a division of application Ser. No. 807,l05, filed Mar. l3, 1969, and now abandoned.

This invention relates to an improved method of electro depositing gold on electrical devices, such as gold plating electrical conductors, a novel electrolyte used in practicing the method of electro depositing gold as well as a novel salt composition for use in an electrolyte solution or bath. Electrical devices improved with electro deposited gold thereon are within the scope of this invention.

A wide variety of electrolytes are used as solutions for plating gold therefrom. As it is well known, electrolyte solutions used for this purpose are generally classified in three types based on a pH scale. The first type is an alkaline electrolyte solution. This electrolyte contains gold as potassium gold cyanide in solution and additional amounts of potassium and sodium cyanide. Electroplating from a bath operating with this solution is carried out at a pH of 8 and higher.

As a second type, an acid gold electrolyte solution is well known. It contains gold in a form of complex cyanides, and it has additional salts dissolved in the solution. These salts are derived from organic acids such as citrates, acetates, lactates, etc. Electro depositing from this bath is carried out at a pH below 6.

As a third type of electrolyte solution, a neutral gold electrolyte is employed. It is a buffered solution whose pH level is kept from 6 to 8. Although it is not truly a neutral bath over this pH range, it has been designated as such in the art.

A number of variations are known of these three basic electrolyte bath compositions. For example, additives used as complexing agents are often employed. While these additives offer a solution to certain problems, these same additives, together with the gold salts, cause other problems.

A very complex and as yet poorly understood electrochemical interaction of the additives with eachother is the reason for a wide variation in results. Further, not only dissolved gold containing salts interact with the additives, but also the variation in the anodic and cathodic reactions as these occur in the bath produce different results. These various phenomena render the chemistry of electroplating gold from an electrolyte solution an art with predictability being very difficult, if not impossible.

Hence, when the electrolyte components are changed or substituted, many balancing considerations apply where sacrifices in one variable must be made for a gain in another variable. However, these balancing considerations are invariably empirical. As an electrolyte bath consists of a number of components, the permutations and combinations and the effects of each additive on the electrolyte behavior and the obtained product are manifold; and, consequently, the outcome in the method of plating and the final product, while often acceptable in one respect, are completely unacceptable in some other respect. Further, as it is well known, various electrolytes or baths based thereon can only be operated over a narrow temperature, pH, and

set dissolved salt composition range, as the variations in these may cause gold to be reduced and precipitated from the bath solution. Still further, in operating the bath, unwanted products are formed from trace impurities in the electrolyte; these impurities interact with the electrolyte and/or the gold being electro deposited such as forming alloys or becoming occluded in the electro deposited gold.

These impurities often interact with a base metal or the electroformed gold in the operating environment to which the gold plated device is to be subjected. As a consequence, the end product, while initially acceptable in appearance, fails in one or more of the basic requirements which are hardness, ductility, wear, continuity of film or porosity, oxidation resistance, along with other characterizing properties used to evaluate electro deposited gold and which measure the quality of the deposited layer on an electrical conductor and- /or deposit such as conductivity of the composite. coloration, density, smoothness, purity. nobility, etc.

When gold in various compositions is deposited from an electrolyte such as in a standard electrolyte bath such as when employing a rack or barrel method, or when gold is deposited selectively such as when employing a strip line, jet or belt method, generally it deposits in two crystalline forms. In the so-called alloyed gold, i.e. containing co-deposited additives, the form is typically laminar with the laminae or striations of the crystals parallel to the base on which it is deposited and visible at magnifications of about to 200 times. 'With high purity golds a columnar form is more typical with the crystals forming columns perpendicu-' lar to the base on which it is deposited, visible at about the same magnification. More rarely, gold deposits in a random crystalline form and still more rarely in arandom, fine grained crystalline form.

Laminar and columnar gold crystals are formed from the conventional electrolytes, and these forms are fairly well known, however, it is impossible to predict when fine grained, bright, randomly oriented crystals will be formed from an electrolyte solution, and what electrolyte solution will produce bright, ductile, hard and randomly oriented fine grained crystals. Moreover, it is substantially impossible to predict when the fine grained crystals will form bright gold.

-' For the above reasons, the compounding of suitable salts and the use of these in an electrolyte bath in terms of the bath efficiency and final product properties is still an art beyond routine results achievable with conventional electrolytes suitable for gold plating. Thus; the evaluation of any novel electroplating bath'composition must be based on the electrolyte composition, the method of employing the novel electrolyte bath, and physical properties of the electroformed gold and products made therefrom, any of which may result in a novel combination in itself.

A novel electrolyte salt composition has been found to be suitable for depositing gold from an electrolyte solution. This electrolyte salt composition when used in an electrolyte solution in a particular manner has resulted in a novel method for depositing gold, characterized by high efficiencies, fast plating times, and absence of detrimental side reactions; and more importantly, it produces a novel electro deposited or formed gold product, novel articles of manufacture based on the novel electro deposited or formed gold, which articles possess exceptionally advantageous properties, such as brightness, hardness, e.g., up to 260 Knoop units, foil strength, ductility, wear, smoothness, solderability, etc. These properties in the final product are achieved despite the presence of a heretofore unwanted component in the plating bath which component in fact is now incorporated in the electro deposited gold layer by means of a heretofore note employed complexing agent.

According to the invention, an electro deposited or formed gold has been obtained which is bright. hard and ductile and which consists of randomly oriented, fine grain crystals. This gold displays brightness, hardness, and ductility properties, to name only a few, which properties are contributed by the co-deposited arsenic, the last via a particular complex, e.g., thioarsenic (lll) complexes and/or tris (thio sulfato) arsenic (Ill).

The above mentioned arsenic complexes contribute to the unexpected results in a manifold manner. First, these complexes elevate the reduction potential with all the attendant advantages, and second, the complexes prevent the oxidation of As to As*'**. Additionally, when using these complexes the electroplated or electrodeposited gold possesses the exceptional properties previously mentioned.

Suitable thio or thio sulfato arsenic complexes are those formed from arsenic and a thio compound such as double mercapto group containing compounds such as dithio lower alkylenes, e.g., l, 2 ethanedithiol, dimercapto alkylene ethers, dimercapto alkylene glycols or polyglycols, or alcohols such as, lower alkylalcohols, e.g., 2, 3-dimercapto propanol; rubeanic acid, etc.; further tris (thio sulfato) arsenic complexes are formed from precursors such as alkali thio sulfates, e.g., sodium thio sulfate and arsenic etc. These complexes, however, must be soluble or solubilizable in the electrolyte at the operating conditions.

Without espousing any theory, these complexes contribute to the electrolytic deposition reaction by possibly some catalytic action, perhaps by providing arsenic in a form which does not escape as an arsine gas but rather as an alloy forming component under proper temperature and current density conditions. It is believed that 0.1 but better 0.2 to 1% of weight of arsenic in the gold obtained from an electrolyte solution by electro deposition or electroforming in the specified manner produces a novel form of crystalline gold as evidenced by many of the physical and chemical characteristics of this product as further discussed herein.

For example, the hardness of a gold foil produced according to the novel method when measured in Knoop units at 25 gram load shows a range of 190 to 250, which hardness, together with the exceptional brightness and ductility has heretofore not been obtained.

Further, the Tafel slope of the novel electro deposited or formed gold is almost equivalent to pure gold when measured against pure gold electrodes in a 0.1 molar ammonium chloride solution used in this test.

The above properties such as brightness are exceptionally well displayed when the amount of arsenic in the electrodeposited gold is 0.2 to 0.5 per cent by weight and when the deposition is properly carried out. In the deposit, the balance is gold with trace impurities normally associated with electro deposition of gold. As the amount of arsenic increases in the gold deposit, the properties also vary, for example, at concentrations above 0.9% by weight, the grain structure starts to change and at about 1.0% by weight of arsenic, crystal structure tends towards the columnar type.

Consequently, on basis of the amount of arsenic by weight and the crystalline structure, the present electro deposited gold can be delineated from the prior art electro deposited gold such as by grain structure, brightness, hardness, ductility. Although other fine grained, crystalline gold forms have been observed, these are neither based on arsenic, nor as bright and hard, nor ductile at the hardness levels herein, nor as noble when compared for polarization behavior in respect to slope characteristics based on Tafel equation, i.e., Tafel slope.

Surface smoothness of the novel gold deposits is also exceptional. In an electron microscope at a magnification of 32,000 times and a photographic enlargement of up to 80,000 times, these deposits appear exceptionally smooth based on the definition employed in the art.

Additional, exceptional properties of the novel gold deposits which are based mostly on performance are abrasive resistance, wearability in use of a gold plated article, oxidation resistance, easy wetability for soldering, capability of forming thick electro-deposits, and good throwing power, e.g. 37% as determined in a Haring cell.

In reference to electrical contacts having on the contact surfaces the novel electro deposited gold, the product specifications govern these devices for their acceptability. Specific geometries employed for the specific contacts have characteristic wear cycles. When electrical devices are compared which have contact surfaces plated with the novel gold deposits to those plated with gold having a similar hardness produced from a commercially available electrolytic formulation containing gold, cobalt, and indium, it has been found that on statistical bases the same connectors which have previously been barrel plated with the novel gold deposit have outlasted and outperformed the similarly plated prior art connectors after repeated work cycling (insertion-withdrawal) e.g., when measured at initial low level contact resistance and rated current resistance and after 250 to 500 work cycles at l and 2.5 milliamps DC. and 5 amps A.C. At the same time the novel gold plated connectors have out-performed the prior art plated connectors based on the same test after repeated work cycling and when measured as final contact resistance for durability and corrosion after repeated temperature cycling.

lmproved results based on these tests bespeak the ability of the connectors to perform especially in low voltage level circuitry for extended periods of time. Other equally important properties which the novel gold plated connectors possess are crimpability, i.e. it relates to ductility, scratch resistance, gold deposit porosity, chemical resistance, excellent solderability etc. The last property is measured by means of a dip method comparing solder covered area and solder bare area; it can also be measured by ease of spreading of a certain amount of solder and the surface area which a certain amount covers and the contact angle of this so]- der with the plated surface.

If the novel electro deposited gold is used to produce heavy deposits, it still displays the fine crystalline structure which is rather unexpected. lt is also rather unexpected that while commonly employed hardening and brightening agents normally tend to produce an oxide film either under accelerated aging conditions or in use TABLE 1 ELECTROLYTE BATH COMPOSITION Wei ht of Components Electrolyte Bath Components For nit Volume of Water Monobasic potassium phosphate (KH PO 60 g/l 8 oz/gal Potassium citrate (K C -H O H 60 g/l 8 oz/gal sequestering agent such as disodium ethlenediaminetetrace tate dihydrate (Na EDTA .ZH O) .25 g/l .033 oz/gal Gold as potassium gold cyanide [K Au(CN) (the amount used is 10.7 to 14 g/l on basis of elemental gold) 1.3 to 1.5 tr. oz/gal Arsenic as sodium arsenite .02 to .04 g/l lNaAsO (the amount used is on basis of elemental arsenic); and as complexing agent therefor, sodium thiosulfate .0026 to .0052 oz/gal 5 g/l .66 oz/gal In a bath solution used herein, the total quantity of the specified amounts of each component is added per unit volume of the water according to the units employed. The sequestering agent is an optional additive and is employed as a safeguard against contamination.

In respect to the ranges for the active components, gold in the form as defined above is used ranging from 10.7 to 14 grams per liter or 1.3 to 1.5 troy ounce per gallon, and these are the desired ranges, with the pre ferred amount being about 12.3 g/l. Arsenic is desirably used ranging from 0.02 to 0.04 grams per liter or 0.0026 to 0.0052 ounces per gallon, preferably about 0.03 g/l.

1n operating the bath, it has been found that temperature ranging from 130 to 160F can be usefully employed. An optimum temperature is 140F. If lower temperatures are used, then it has been found that the brightness of the heavy deposits decrease without loss of hardness and efficiency. Thus, at a temperature of 80F the deposits become softer (about 155, Knoop hardness number) and burnt. As the present electrolyte bath is of so-called neutral bath type previously discussed above, a suitable pH range is from 6.0 to 6.5. A pH of 6.2 represents the optimum condition. Specific gravity of this bath is at least Be. The sodium (thiosulfate)-As(lll) complex is replenished in the electrolyte bath by adding a solution of 100 g/l of sodium thiosulfate (Na S O .5H O) and 17.4 g/l of sodium arsenite (NaAsO and bringing up the bath to the specified electrolyte composition.

As the non-consumed anode material, carbon or platinum is used. Equally inert materials may be also employed.

According to the invention, rack, basket, strip or selective plating methods are suitable for obtaining the novel gold form. The use of a strike is recommended, since the electrolyte bath will immersion deposit gold on nickel, copper or brass in less than one minute.

The above described gold is commonly deposited on a metal base. Metals suitable for this purpose and also useful as electrical conductors are such as copper and its alloys, iron and its alloys, nickel and its alloys, aluminum and it alloys, etc.

Curren density for a conventional rack plating method is up to about 10 amperes per square foot (asf); at higher current densities in a rack plating method, the grain structure may change.

At a rack current density of 5 asf heavy deposits are dull and at 15 asf heavy deposits are burnt; there is negligible change in efficiency or hardness of the deposit. For a barrel plating method, current density is about 3 asf. Higher current densities are obtainable when different processes are employed, e.g., up to asf and higher if the previously mentioned arsenic complex formers, which also function as arsine suppressants, are employed in the bath.

Agitation of the bath during the plating operation is carried out by mechanical means in a vigorous fashion.

The density of the deposit obtained, when employing a bath solution of the composition given in Table 1, was 19.0 grams per cubic centimeter or 31.2 milligrams per 0.0001 cubic inches. A hardness of to 250 Knoop hardness units with a 25 gram load was obtained.

In the gold depositing art, the surface appearance of a deposit is classified commonly as either bright, semibright, or matte. In accordance with this invention, rack plating work is bright to a thickness of at least 1.5 mils, while barrel plated work is bright to a thickness of at least 0.2 mils.

As mentioned before, the novel method of operating a bath is characterized by excellent efficiency either on percentage basis or deposition rates. Representative efficiency figures for the electrolyte bath of the above composition are illustrated below:

TABLE 11 EFFICIENCIES WITH ELECTROLYTE BATH OF A MAKE UP ACCORDING TO TABLE I Efficiency The efficiency in percent is defined as the amount of gold deposited in comparison with the theoretical efficiency for pure gold which is 123 mg/amp minute.

When leaf type electrical connector articles were plated in the same bath, a plating rate to achieve 0.1 mils deposition was achieved in a rack plating method in 3.6 to 4.0 minutes at 10 asf at the above-identified optimum conditions while vigorously stirring the electrolyte bath by mechanical means; in barrel plating, the same rate for the same deposit thickness at 3 asf was achieved in 14.5 to 15 minutes.

Electrical devices which can be suitably electroplated with the novel gold composition according to a method disclosed herein are items such as a crimpable or solderable electrical conductor, a coaxial connector, microminiature connections for integrated circuits, solid state devices, etc.

The structure of the novel gold deposit changes when certain impurities are found in the electrolyte. For example, copper, iron, and nickel are commonly encountered impurities. While copper, iron and nickel as an impurity(ies) do not adversely affect bath efficiency or properties of the deposit, iron does effect a change in the crystalline structure of the deposit at levels as low as lOmg/l of electrolyte solution. At this concentration,

the structure may be partially of randomly fine crystals and partially of columnar crystals. Further, while copper and nickel at low current density values, i.e. when rack plating at asf, and concentrations up to 100 mg/l, do not change the crystalline structure and prop- 5 erties at high current density values, e.g. at 10 asf or above, the structure may be laminar for copper or columnar for nickel. These observations were made when plating with the composition in Table l at [F and at 10 asf. However, these impurities do not reduce gold from the electrolyte bath at the operating conditions.

For high speed plating operation, the following electrolyte composition has been found to be very suitable. Composition and operating conditions of an electrolyte bath and properties of the gold deposit are illustrated below.

TABLE [II ELECTROLYTE COMPOSlTlON, OPERATING CONDITIONS AND PROPERTIES OF ELECTRO 20 DEPOSlTED GOLD IN A HIGH SPEED PLATlNG OPERATION The above-described properties of the novel gold form were determined according to the procedures for gold plated products set out in ASTM B488-68.

The term an electro deposited gold article is meant to cover both an electroplated article and and electroformed gold article of which the latter is an article which is built up to the desired dimensions by gold deposition from an electrolyte. The term a heavy gold deposit signifies deposited gold of a thickness in excess of 1 mil. In reference to a rack plating method high current density signifies values above 10 asf, i.e. up to 50 asf. A low-stress gold deposit is defined herein as being ductile according to the ASTM test indicated above; and wherein an electro deposited gold foil. when removed from a base metal upon dissolution of the base is characterized by absence of failure by bending and unbending over a 0.32 dia wire through angular degrees.

What is claimed is:

1. An electro deposited bright, ductile gold alloy of arsenic having 0.1 to 0.9% of arsenic by weight on basis of the alloy, having a Knoop hardness of up to 260 Knoop units and a Tafel slope substantially equivalent to pure gold, the amount of gold being the balance and including trace impurities associated with said electro deposited gold alloy.

2. An electro deposited, bright, ductile gold alloy of arsenic having a fine grained randomly oriented crystal structure having 0.2 to 0.9% of arsenic by weight on basis of the alloy, 21 Knoop hardness of up to 260 Knoop units, and a Tafel slope substantially equivalent to pure gold, the amount of gold being the balance and including trace impurities associated with said electro deposited gold alloy.

3. An electro deposited gold alloy according to claim 2, wherein the arsenic is present from 0.2 to 0.5% by weight and wherein the Knoop hardness is from to 250 Knoop hardness units.

4. An electro deposited gold alloy according to claim 1, and wherein said alloy contains from 0.13 to 0.9% of arsenic and wherein the Knoop hardness of said alloy 

1. AN ELECTRO DEPOSITED BRIGHT, DUCTILE GOLD ALLOY OF ARSENIC HAVING 0.1 TO 0.9% OF ARSENIC BY WEIGHT ON BASIS OF THE ALLOY, HAVING A KNOOP HARDNESS OF UP TO 260 KNOOP UNITS AND A TAFEL SLOPE SUBSTANTIALLY EQUIVALENT TO PURE GOLD, THE AMOUNT OF GOLD BEING THE BALANCE AND INCLUDING TRACE IMPURITIES ASSOCIATED WITH SAID ELECTRO DEPOSITED GOLD ALLOY.
 2. An electro deposited, bright, ductile gold alloy of arsenic having a fine grained randomly oriented crystal structure having 0.2 to 0.9% of arsenic by weight on basis of the alloy, a Knoop hardness of up to 260 Knoop units, and a Tafel slope substantially equivalent to pure gold, the amount of gold being the balance and including trace impurities associated with said electro deposited gold alloy.
 3. An electro deposited gold alloy according to claim 2, wherein the arsenic is present from 0.2 to 0.5% by weight and wherein the Knoop hardness is from 190 to 250 Knoop hardness units.
 4. An electro deposited gold alloy according to claim 1, and wherein said alloy contains from 0.13 to 0.9% of arsenic and wherein the Knoop hardness of said alloy is at least
 150. 